Apparatus for producing films in accordance with the blown tube process

ABSTRACT

The invention provides a novel apparatus for producing, by blown tube process, novel film having improved balanced characteristics. The film is characterized by crystallites dispersed throughout the film in amorphous areas and which is substantially free of crystalline formation, as well as having other improved mechanical properties. The apparatus employs a first cooling chamber for acting on the molten tube following extrusion for cooling the tube to a temperature above the crystalline formation temperature, an annealing chamber located downstream from the first cooling chamber and a second cooling chamber located downstream of the annealing chamber to initiate solidification of the tube.

United States Patent Saint Eve et al.

APPARATUS FOR PRODUCING FILMS IN ACCORDANCE WITH THE BLOWN TUBE PROCESSInventors: Daniel R. Saint Eve, Thornhill,

Ontario; Ajit Kurnar Bose, Don Mills, Ontario, both of Canada Assignee:Leco Industries Limited, St.

Laurent, Quebec, Canada Filed: May 11, 1973 Appl. No.: 359,563

Related US. Application Data Continuation-impart of Ser. No. 79,754,Oct. 12, 1970, Pat. No. 3,754,067.

US. Cl. 425/72; 264/95; 425/326; 425/388 Int. Cl B29c 25/00; B29f 3/08Field of Search 425/326 R, 387, 383, 72, 425/445; 264/95, 98, 99

References Cited UNITED STATES PATENTS 12/1970 Ebert et a1. 264/95 XJune 10, 1975 3,568,252 3/1971 Masuda et al. 264/95 X FOREIGN PATENTS ORAPPLICATIONS 6,707,257 11/1967 Netherlands 264/95 PrimaryExaminer-Andrew R. .luhasz Assistant Examiner-William R. BriggsAttorney, Agent, or Firm-Lawrence 1. Field [57] ABSTRACT The inventionprovides a novel apparatus for producing, by blown tube process, novelfilm having improved balanced characteristics. The film is characterizedby crystallites dispersed throughout the film in amorphous areas andwhich is substantially free of crystalline formation, as well as havingother improved mechanical properties. The apparatus employs a firstcooling chamber for acting on the molten tube fol10wing extrusion forcooling the tube to a temperature above the crystalline formationtemperature, an annealing chamber located downstream from the firstcooling chamber and a second cooling chamber located downstream of theannealing chamber to initiate solidification of the tube.

12 Claims, 2 Drawing Figures 1 APPARATUS FOR PRODUCING FILMS INACCORDANCE WITH THE BLOWN TUBE PROCESS This application is acontinuation-in-part of my copending application Ser. No. 79,754, filedOct. 12, 1970, now US. Pat. No. 3,754,067, relating to improvements inBlown Tube Production.

The invention relates in general to certain new and useful improvementsin thermoplastic film and tubing and more particularly, to film andtubing having amor phous areas with crystallite formation therein.

In the present specification, for ease of understanding and descriptionof and illustration of the invention, reference will be made topolyethylene as a typical extrudable film-forming thermoplastic resin,since it is one of the most common materials in use today for suchpurposes; however, it will be understood that the invention is notlimited thereto, as described hereinaf- Film and tubing of polyethylene,and other similar materials, finds many uses in various and diversifiedfields, eg the packaging art, the construction industry, etc. Dependingon its particular use, different properties are required; for example,in the packaging field, some uses require the material to be shrinkableeg in shrink packaging where a wrapper or label is applied to acommodity so that it assumes the contour of the commodity after a filmortube-shrinking treatment. Other requirements for film or tubing, eitheralone or in combination with the shrink characteristics, include highgloss (optical) and/or low haze (frostiness), usually together with goodtensile strength, good elongation, good secant modulus and good impactstrength properties.

The above characteristics are well defined in the art and brieflysummarized, the term shrink or shrinkable describes film or tubingcapable of being shrunk in overall area upon suitable activation e.g.normally by heat treatment (which is generally carried out after thefilm or tubing is placed in situ e.g. about an article or commodity).

Low haze characteristics of a product are highly desirable, as otherwisethe transparency of the tube or film is impaired making it of limitedvalue. High gloss is desirable in other products to provide acommercially desirable appearance. The tensile strength of film andtubing must also be sufficient to withstand normal handling in usewithout rupturing. The secant modulus, tied in with the impact strengthcharacteristic, must be sufficient so that the film or tubing has goodstiffness, but yet is flexible enough to permit use for packaging, etc.In the case of impact strength, the film or tubing must have sufficientcharacteristics to enable it to be handled and used without rupturingdue to brittleness, and for being *contoured about a product.

Several of the above characteristics are influenced by the fact thatpolyethylene resin, when manufactured, as with other similar materials,possesses a degree of crystallinity which varies according to the typeof process (e.g. batch, J-type reactor) by which it was produced. Inhomopolyethylene of a low density type, the degree of crystallinity willbe, e.g., between 50 60%.

Conventionally, a large percentage of film and tubing is made by what isknown in the art as the blown tube" or blown bubble" method. The actualdetails are well known to those skilled in the art and reference may behad to, e.g., Canadian Pat. No. 460,963 illustrating such a method.Briefly, the method involves extrusion of an extrudable film-formingresin, using an extruder, through an extrusion die to form a bubble ortube, which is then blown up (using a differential air pressure betweenthe interior of the bubble and atmosphere) to a desired size. The bubbleor tube may be slit after solidification to form film (sheeting). 1n theconventional process, the molten bubble after extrusion is acted on by asingle cooling means. ln other conventional processes, a pair of coolingmeans may also be employed, as for example in US. Pat. No. 3,568,252.

The properties of blown tubing or film of polyethylene vary considerably(depending on various extrusion conditions e.g. melt temperature). Evenoperating under optimum conditions for any given property de sired, theproperties of e.g. low density polyethylene may be generally stated toresult in a haze percentage off: to 8% (measured by ASTM test D-1003-61; excellent being 0 5%, good being 5 10% and poor being 10% andhigher); gloss percentage of to (measured by ASTM test D 523-53T;excellent being 90 good being 75 89%, and poor being 7471 and lower);tensile strength averages of between 2000 to about 2800 psi (based on anaverage of machine direction and transverse direction; values of 2000 to2900 psi being excellent; good being 1700 to 2000 psi, and poor beingbelow 1700 psi), a secant modulus of about 30,000 (excellent being29,000 to 31,000); impact strengths of about 60 grams/mil (1 milthickness) (excellent being 56 64 grams/mil) and shrink characteristicswhere the ratio of machine direction (MD) to transverse direction (TD)is between about 8:1 to about 5;1, with averages of 70% shrinkability inthe MD and 20 to 30% in the TD. With the tensile strengthcharacteristics (like the shrink feature) the blown tube or filmnormally has a far greater strength in the machine direction as opposedto the transverse direction, eg 2800 psi compared to 2300 psi.

[t is known to those skilled in the art that improved glosscharacteristics (within 90 to 105%) and haze (4 5%) can be obtained byincreasing the melt temperature of the extruder and by increasing thefrost line height of the blown tube; however, by doing so, there is aresulting large drop in the tensile strength, secant modulus and theimpact strength characteristics of such films. Conversely, the tensilestrength, impact strength and secant modulus of the product can beincreased but only at the expense of the haze characteristics (tobetween about 10 to 20%) and gloss characteristics (between 75 to 89%).

Conventional blown tubing or film likewise can be produced, according toexisting technology, where there is a vast differential between theshrink characteristics in the transverse direction as opposed to themachine direction (the direction in which the tube is extruded from theextruder and wound up). This is due to an inherent characteristic of theblown tube process, in that the molten tube must be stretchedlongitudinally (in the machine direction) between 5 to 8 times thatwhich the bubble is transversely expanded (using a differential airpressure) in order to obtain a stable bubble during production; andfurther by using a blow ratio of at least 1.5: 1. Consequently, suchfilm or tubing when shrunk will preferentially shrink in the machinedirection, which of course necessitates corresponding allowances indimensions for packaging purposes, etc. Still 3 further. it is verydifficult to provide shrink film con taining indicia which when shrunk.will produce a prod uct having clear and legible indicia thereon.

Optimum characteristics for versatile tube or film would desirably be ahaze percentage of between about to 5%. gloss characteristics of 90 tol057r. tensile strength average of about 3.000 psi (with the strength inboth the machine direction and transverse direction being substantiallyequal). a sccant modulus of about 30.000 and an impact strength of atleast 60 grams/mil (l mil thickness].

Likewise, optimally. the shrink characteristics offilm or tubing shouldbe such that the machine direction and transverse directioncharacteristics are substantially identical.

Many attempts and a large amount of research have gone into trying toimprove and balance the properties of film and tubing withoutsacrificing one or more other properties for example, post treatments offilm have been attempted, but generally the tensile strength is loweredas a result or other properties are sacrificed. However. in general.none of the processes are very successful.

With this invention, Applicant has developed a very simple. economicaland practical apparatus and process for producing film and tubing, usingthe blown tube process, which overcomes the disadvantages of the priorart processes and at the same time provides improved properties in thefilm or tubing.

According to one aspect of the present invention, film or tubing can beproduced using the apparatus hereof so that an extrudable film-formingthermoplastic resinous material is extruded to form a molten tube orbubble by the blown tube process, and wherein the molten extruded tubeis blown up to a desired size following extrusion. In accordance withthe improvement in the apparatus utilized in the making of the tube orfilm. the apparatus allows for acting on the molten tube followingextrusion, to cool the molten tube to a temperature above thecrystallization formation temperature of the material. The molten tubeis subsequently annealed for a time sufficient to permit the material toform amorphous areas therein. and thereafter the resulting moltenannealed tube is again cooled to solidify the tube.

According to a further aspect of the present invention, in an apparatussuitable for producing film or tubing by the blown tube method. andwherein there is included at least one extruder, an extrusion die andmeans for causing a molten extruded tube to be blown up to a desiredsize following extrusion, there is provided the improvement comprisingfirst cooling means adapted to cool a molten tube of material extrudedfrom said die. said cooling means being adapted to be mounted inoperative relationship to said die whereby a molten extruded tube iscooled by said cooling means after extrusion from said die. an annealingchamber in operative association with said first cooling means in whicha cooled molten tube is annealed. and further cooling means in operativeassociation with said annealing chamber for further cooling the annealedmolten tube.

In accordance with the teachings of the present invention. it ispossible to provide new polyethylene products; which in one embodimentis solidified amorphous polyethylene film or tubing having crystallitesdispersed throughout said film in said amorphous areas,

the film be :ig substantially free ofcrystallinc polyethylvine. and in astill further embodiment an extruded polyethylene film or tubing havinga more balanced machine direction:transverse direction ratio e.g.between about 21! to about 1:]. the film being further characterized bya tensile strength average of the machine and transverse directions ofat least 2800 psi, haze properties as measured by ASTM D-l003-6l of lessthan about 4% and gloss characteristics as measurcd by ASTM D-523-53T ofat least [15.

Following the teachings of the present invention. there may be obtainedfilm and tubing having improved properties through the application ofrelatively simple and economical techniques as compared to the pro posedsolutions of the prior art. The present invention lends itself toadaptation on existing blown tube type of equipment provided withmodifications as described herein. The process of making the film inaccordance with the present invention may also be carried out at ratesof film and tubing production, while maintaining the improvedcharacteristics of the products, superior to any other proposed systemfor obtaining products by the blown tube method. having only one or twoof the characteristics desired in the product. A still further advantageof the present invention is that no extra steps are required to producefilm having improved charac teristics as in prior art proposals.

Without being limited to any theory, the present invention provides forthe treatment ofthe molten mate rial to first cause the formation ofcrystallites and permits the formation of amorphous material surroundingthe crystallites whercafter the molten film is cooled to solidify thematerial substantially without any crystalline formation. During theformation of the crystallites most of the expansion of the molten tubeis carried out which aids in the formation of a substantially uniformdistribution and spacing of the crystallites which appear to haveamorphous areas surrounding the crystallites i.e. during expansion, thecrystallites are stretched in both machine and transverse directions.

The step of crystallite formation is preferably substantiallyaccomplished during the first cooling step, and is substantiallycompleted following annealing of the molten material. In some cases. itmay be desirable to cause crystallite formation following initialcooling of the molten material Le. during the annealing stage. in whichembodiment, the annealing stage will also include slight cooling action.

in all cases. the temperature of the molten material during the initialcooling and annealing stages is maintained above the crystallineformation temperature of the material being extruded, and desirablywithin a temperature range of from about 30F. below the temperature atwhich the material is extruded but above the crystalline formationtemperature of the material. As will be appreciated, the temperature mayvary according to the specific type of material being used in theprocess. but generally speaking for polyethylene and other similarpolymers or copolymers, the material will be cooled to between about20F. to about lOOF. from the temperature at which the material isextruded. The crystallite formation temperature that is the temperatureat which crystallites form in the material will vary depending on theparticular material being used. These temperatures for differentmaterials are known to those skilled in the art or may be readilydetermined. In the ease of low density polyethylene,

temperatures conducive to formation of crystallites, using a melttemperature of 320 to 350F. are between about 290 to 320F., which willvary depending on the specific type of resin used as the startingmaterial.

The initial cooling step may be carried out by any suitable means, asfor example by an externally applied gaseous cooling medium (typicallyair. having an appropriate temperature). solid cooling means cg. acooling ring again having the appropriate temperature, etc. As will beunderstood by those skilled in the art. the temperature of the firstmeans used to cause cooling in the first cooling step will varydepending on fac tors such extrusion speed of the film, melttemperature, etc. Preferably, the first cooling step is carried outusing air as a gaseous cooling medium and under conditions such as aminimum volume of air having a medium velocity is employed. Followingthis procedure, the gaseous cooling medium will also impart stability tothe bubble.

During the first cooling step, and employing the conventional blown tubeprocedures, the molten tube will partially expand during the step. Thus,crystallites formed in the material will be subject to machine andtransverse direction expansion.

The second step of annealing molten material is preferably carried outso that there is a minimum of disturbance created on the molten bubble.During the annealing step, heat is removed from around the moltenbubble, which can expedicntly be accomplished by creating a partialvacuum externally surrounding the molten bubble. It has been found thatwhere crystallite formation is permitted to occur during the firstcooling step, the annealing step will permit amorphous areas to form andgrow around the crystallites. However, as pre viously mentioned,crystallite formation may take place in the annealing stage by causing aslight cooling action of the molten bubble.

Desirably, the annealing operation is carried out in the presence of apartial vacuum and with the bubble subjected to a slight cooling action.The annealing step should also be carried out at a point removed fromthe first cooling step in the direction of travel of the molten tube.

It has been found, in accordance with this invention, that by employinga vacuum step in the annealing operation, additional stability isimparted to the bubble as will be hereinafter explained, and furtherthat the bubble is permitted to expand by virtue of the partial vacuumwhich is believed to impart the additional stability.

The term annealing is often used in connection with the treatment ofvarious materials such as glass or certain metals in order to renderthem less brittle, In this way strength is imparted to the material.Generally the annealing is carried out by heating the material and thensubjecting the material to a slow cooling, or otherwise temperingthrough a gradually diminished heat. in this respect the thermoplasticfilms are annealed in accordance with the present invention, even thoughthey are not subjected to the initial heating step. However, bysubjecting the materials in their molten state after the initial coolingto a vacuum or a partial vacuum under the proper temperature conditionsas described herein, and perhaps to a slight cooling effect, the filmsare annealed in that their strength is increased by rendering them lessbrittle. In this stage, only the sensible heat is removed from the filmmaterial. The annealing is carried out in an annealing chamber and theamount of vacuum and/or cooling is carefully adjusted so that only thesensible heat is removed and also to permit the tube to expand to theextent necessary to form crystallites therein after the initial cooling.

In the next step according to the present invention, the annealed moltenproduct is subsequently permitted to solidify preferably by passing themolten tube through a further cooling step, whereby the molten materialis transformed into a solidified tube substantially without theformation of any crystalline structures therein. During this last step,the molten tube is further expanded to the desired size. Preferably thelast cooling step is carried out as a quench step, using a high volumelow to medium velocity gaseous cooling medium, e.g. air. Desirably, thelast cooling step is carried out at a point removed from the annealingstage.

In effect, applicant has found that the process of the present inventionacts as a type of double extrusion process, wherein following extrusion,the molten tube is expanded to a first flow ratio size. wherein themolten tube is cooled to initiate the formation of crystallitcs whilesmoothing out any melt irregularities. Thereafter, following annealingwherein preferably only the sensible heat load is transferred to theatmosphere, the tube is subsequently expanded to the desired blow ratiosize using a fast cooling step. In the final expansion, unlike theconventional blown tube process wherein molten material is extruded andwhich encounters die resistance, the present invention permits expansionunder virtually neutral or relaxed melt conditions, whereby the finalblow ratio is a combination of the initial and final expansion stages.

The process of the present invention is preferably carried out in anenclosed area whereby undesirable influences on the molten tube areeliminated. To this end, the first cooling step, the annealing step andthe final cooling step may be carried out in a closed housingsurrounding the molten tube at the appropriate positions.

Applicant has further found that desirably, the combined minimum blowratio that should be employed for best properties in the extruded filmis about 2.5, while blow ratios of 3.5 impart better balanced propertiesin the products while permitting higher outputs having good bubblestability during production.

It will be understood that during the first and final cooling steps,there may be employed more than one cooling stage to impart the desireddegree of cooling to the molten bubble. However, this is not normallynee essary as a single cooling stage can usually be employed toaccomplish the desired cooling.

In carrying out the process of the present invention the apparatusaccording to a further aspect may include any suitable first coolingmeans adapted to cool the molten tube, following extrusion, to thedesired temperaturev conventionally, such cooling means comprise meansfor supplying a gaseous cooling medium to the external surface of thetube, with suitable means for controlling the degree (amount) andvelocity of the cooling medium. Desirably, the first cooling means is ofa substantially similar construction to that hereinafter described withrespect to the second cooling means. Suitable means may be provided formounting the pri mary cooling means to the extrusion device, preferablyat a point slightly removed from the point at which the molten materialis extruded an annular tube i.e. from the extrusion die.

The annealing chamber may be in the form of an enclosed chamber havingmeans for withdrawing the sensible heat load from the atmospheresurrounding the molten tube. In a preferred embodiment, the annealingchamber comprises an annular housing surrounding the tube with aperturestherein adapted to permit the passage of the molten tube through thechamber, and with means for creating a partial vacuum in said chamber topermit the sensible heat load to be removed. it will be understood thatthe diameter of the apertures ofthc annealing chamber will be sufficientto encompass the annular tube, and preferably V2 to IV? inches greater.Any suitable means may be employed for drawing a partial vacuum in theannealing chamber, such suitable means being. for example, an air pump.The air pump may be connected to the annealing chamber by means of oneor more conduits. Preferably the air pump will have control to permitgreater or lesser amounts of air to be withdrawn from the annealingchamber to allow for variances in the size and type of tube or filmproduction.

The length of the annealing chamber may vary, and if desired, thehousing may be made adjustable to compensate for various dwell times ofthe molten tube. The dwell times will be determined by the type ofmaterial being employed, the diameter of the tube, etc., and may bereadily ascertained by those skilled in the art. Preferably, theannealing chamber is mounted at a position slightly spaced from thefirst cooling means, and to this end, any suitable mounting means may beemployed for positioning the annealing chamber in operative relationshipto the first cooling means.

A second cooling means is in the form of an annular zone, preferablybeing a continuous circumferential zone surrounding the molten tube at apoint slightly spaced from the annealing chamber and in operativerelationship therewith to permit a molten annular tube to passtherethrough. To this end, according to a preferred construction, thesecondary cooling means comprises a housing having upper and lowerapertures therein of a diameter sufficient to permit a molten tube topass thercthrough, means for supplying a source of a cooling gaseousmedium to said housing, and means within said housing for directing saidgaseous cooling medium preferably in the direction of movement of theannular molten tube. Desirably, the last-mentioned means is adjustableto permit variances in the direction of the gaseous medium onto thetube. Suitable means may be provided for mounting the second coolingmeans as, for example, brackets, or the like.

The source of the gaseous cooling medium may be provided for byconventional means, e.g. compressed air, which may be refrigerated ifdesired. The compressed air may be provided at varying velocitiesranging from I000 to 4000 feet per minute. Most desirably, in the firstand second cooling means the gaseous cooling medium is supplied to theannular housing in the form of a circumferential ring, by means of aplurality of individual flow streams, to form a substantially continuousannular curtain of the cooling medium.

The type of equipment used in the extrusion of the film-formingextrudable resin and for maintaining the bubble in a blown condition,may be any suitable equipment known in the art for this purpose. In thecase of single tube extrusion that is non-laminated films, a singleextruder is required to feed a molten resin to an annular extrusion die.The extrusion die may be a single channel conventional type oralternatively, may be a multi-channel type adapted to form a coextrudcdtube consisting of two or more layers of filmforming extrudable resin.In the latter type of die arrangements, a single extruder with dual feedlines may be employed, or two or more extruders feeding different typesof resins can also be employed to produce a co-extruded product havingdifferent properties depending on the different types of resinsextruded,

The products of the present invention have superior properties to thosewhich could otherwise be produced on the conventional blown tube type ofprocess and apparatus. It is now possible to produce much betterbalanced" film and tubing having excellent haze and gloss percentages,tensile strength averages, secant modulus and impact strengthcharacteristics without sacrificing an improvement in one property foranother. Moreover, the characteristics of such shrink film are improved.In the case of polyethylene of low density type, it is now possible toproduce tubing or film having balanced properties where haze percentagesof 3 to 4% (measured by ASTM D-l00-3-6l) may be obtained, glosspercentages of between I l5 to I30 (measured by ASTM D-523-53T), tensilestrengths between 2800 to 3000 or more and impact strengths of between68 to 70 grams/mil (for 1 mil film), and shrink characteristics wherethe machine directionctransverse direction ratio is between about 2:l toabout 1.531.

Moreover, with the present invention, it is now possible to producepolyethylene film or tubing in which the morphology of the material isbased on crystallites surrounded by amorphous areas with substantiallyno crystalline formation therein. The products are further characterizedin that the film or tubing is in a substantially relaxed condition inother words, there are no over-balanced stress-strain factors in adormant form.

In carrying out the process of the present invention, improvedcharacteristics can be obtained in a wide variety of films or tubingproduced from different filmforming extrudable resins. Likewise,improved characteristics may be obtained in films or tubing consistingof two or more layers of co-extruded materials. To this end, the processmay employ typical film-forming thermoplastic resins, or laminates ofone or more of the following resins, polyethylene (all densities),polypropylene, copolymers of polyethylene with other comonomers,copolymers of polypropylene with other comonomers, cellulose acetate,cellulose acetate butyrate, ethyl cellulose, methyl methacrylatepolymer, nylon (extrusion or molding grade) polystyrene, polyvinylformalacetate butyral, copolymers of vinyl chloride and vinyl acetate(Vinylite), polyvinyl chloride (Geon), copolymers of vinyl chloride andvinylidene chloride (Saran) etc.

The resins employed may contain various types of conventional additivesaccording to standard practices in this art, such additives being forexample anti-block, anti-slip, etc. The present invention, as indicated,is particularly suitable for the production of film by the blown tubemethod; however, the tubing may be split during production thereof,whereby continuous film is obtained. The thickness of the extruded tubemay vary according to conventional procedures and requirements typicalthickness ranging from l0.0005 inch and lower to several mils, e.g. 7 to10 mils or more.

Having thus generally described the invention reference will now be madeto the accompanying drawings illustrating a preferred embodiment and inwhich FIG. 1 is a schematic view of an apparatus according to thepresent invention; and

FIG. 2 is a diagrammatic view showing the condition of the molten bubbleas it passes through the cooling and annealing chambers.

Referring now in more detail and by reference characters to thedrawings, S designates an extrusion system constructed in accordancewith the present invention and comprising a supply conduit having achannel 12 therein for receiving molten extruded resin from an extruder,which connects to an extrusion die indicated generally by referencenumeral 14. In FIG. 1, various components of a typical extrusion linehave not been shown for the sake of simplicity, such components, forexample, including the extruder and the wind-up roll, and the like.However, reference may again be made to the aforesaid Canadian Pat. No.460,963 for the details of the blown tube process.

The extrusion die may be any conventional die such as a rotational die,or a stationary die. The size of the die will vary according totechniques and practices in this art. As previously mentioned, the diemay be a coextrusion die adapted to extrude two or more layers of moltenmaterial; and for simplicity only, a single layer extrusion die has beenillustrated.

The die illustrated includes an aperture 15 for receiving the moltenmaterial from the conduit 12 and thereafter divides into an annularorifice channel 16 to form an annular ring of extruded resin at thedischarge end 16a of the die. A conically shaped solid core 18 supportedby a spider". is positioned in the central portion of the die thereby toform the annular channel 16 with the exterior walls of the die.

It will be understood that the molten resin is introduced into the dieunder pressure which may be according to the conventional techniques. Inthe blown bubble method, a source of pressurized air is introduced intoconduit 20 to air line 22 extending to the conical member 18 to directajet of air upwardly which is surrounded by the annular ring of filmbeing extruded.

Spaced from the die 14 at a predetermined vertical height therefrom area pair of rotatable rollers 24 mounted on drive shafts 26, driven bysuitable means. It is at this point that the bubble or tube, inflated bythe source of pressurized air, is collapsed to form a collapsed tube 28which is then wound up on wind-up rollers (not shown).

The inflated bubble, generally designated by reference numeral 30, ismaintained in an inflated condition by adjustment of the air pressureand other factors will be understood by those skilled in this art. Onceextruded from the die, the bubble or tube which is in a moltencondition, solidifies at what is known as a frost-line" in the art,shown by dotted lines 32.

In accordance with this invention, the first cooling step is carried outby providing a first cooling means, preferably in the form of an airring designated by reference letter A. The annealing chamber isgenerally designated by reference letter C and the second cooling stepis carried out by second cooling means, preferably in the form of asecond air ring generally designated by reference letter B. In apreferred embodiment, both cooling means are of a substantially similarconstruction, and in the following description similar referencenumerals designate similar components.

Each cooling means comprises an air ring having a housing 40 forming anenclosed chamber. Housing 40 includes an aperture 42 therein, whichaperture is of a size sufficient to permit a molten extruded blown tubeto pass thcrcthrough, as will be evident from FIG. 1. The housing 40also includes an internal wall 44. otherwise known a chimney. Dependingon the particular constructure of the air ring, the height of thechimney 44 may vary, and may be adjustably mounted on the housing tovary the angle between the bubble and the chimney. In the arrangementshown, the chimney 44 of ring A is commensurate with the height of thehousing, while in the case of ring B, the chimney 44 extends beyond thehousing 40.

Each air ring includes a circumferential air passage designated byreference numeral 46, connected to a source of pressurized air (notshown) which is introduced therein by means of a conduit 49. The airring A also includes an adjustable air discharge opening or nozzle 48and in like manner the air ring B includes a similar nozzle 50. Theadjustment of the nozzle permits the flow of gaseous cooling medium tobe directed at a desired angle onto the external surface of the moltenfilm.

The air ring A may be mounted to the extrusion apparatus by any suitablemeans and in the arrangement shown, the air ring A is connected to thedie. The air ring 8 may be mounted by means of brackets or otherconventional fasteners to the extrusion assembly.

The above-described air rings A and B are of a type known as a Venturi"type of air ring which because of their structure and operation, permita molten bubble to expand outwardly. The expansion of the bubble,particularly during the second cooling step imparts additional stabilityto the bubble to permit an increased extrusion rate. However, the bubblemay be acted on to cause expansion according to various othertechniques.

The annealing chamber, indicated by reference numeral C, is mountedbetween air rings A and B, spacedapart therefrom by a distance of about4 inches. The chamber comprises housing 52, having a conduit 54connected thereto at one end and at the other end to a vacuum sourcee.g. a vacuum pump. The annealing chamber acts to draw the sensible heatsurrounding the molten bubble or tube at this point, so as to permit themolten material to be annealed during this phase of the tube production.In using an annealing chamber of the type illustrated, the tube is alsopermitted to expand whereby the crystallites formed following theinitial cooling step. may be further stretched.

As illustrated in FIG. 2, without the annealing chamber of the typedescribed, the bubble would otherwise assume the configuration shown indotted lines. This further expansion of the bubble also impartsadditional stability. Thus, it can be observed that the bubble ofthermoplastic film will be sujected to a first expansion in the regionof the first cooling chamber A, a second expansion in the region of theannealing chamber C. and a third expansion in the region of the secondcooling chamber B. It can also be observed that relatively littleexpansion, if any occurs during the region of film travel from the firstcooling chamber to the annealing chamber and during the region of filmtravel from the annealing chamber to the second cooling chamber.

EXAMPLE The following example demonstrates the production of a novelpolyethylene product, using the process and apparatus of the presentinvention. In this Example there was employed a polyethylene resin oflow density; the extrusion apparatus employed was that illustrated inFIG. 1. In this case, the extruder was a standard 2% inch 24:1 LDextruder connected to a 5 inch spider type die. The first cooling meanswas as shown in the drawings, having a diameter of about 8 inches; thesecond cooling means was also as shown in the drawings having anaperture therein of approximately 14 inches. The height of the first andsecond cooling means was 5 inches each. The annealing chamber had aheight of 3 inches, and was spaced from each of the cooling rings by adistance of 4 inches. The gap between the annealing chamber and thecooling rings was closed by means of a sheet of film.

The polyethylene resin with the conventional additives therein, e.g.anti-block, was fed to the extruder and a bubble formed according toconventional procedures. The polyethylene had a melt temperature ofabout 340F., and was extruded from the extrusion die at thistemperature. The extrusion conditions were adjusted to provide a filmthickness above the frost line of approximately 1.2 mils average.

The first cooling means was actuated to supply air as a gaseous coolingmedia under medium velocity (of 1000 feet per minute), and was directedagainst the film at a slight angle. During this initial cooling stage,as will be seen from FIG. 2, the molten bubble began to expand, and thetemperature of the molten bubble was lowered to about 295F. At thistemperature, crystallites began to form in the molten bubble, whichcrystallites were stretched due to the expansion of the bubble duringthe first cooling step. Thereafter, the bubble was passed through theannealing chamber whereby the temperature of the bubble maintained atabout 290F., and as will be seen the bubble completed the first stageexpansion whereby it had a blow ratio of approximately l.7.

Following annealing, the molten bubble was then passed through thesecond cooling step wherein cool air was directed at a slight angle ontothe molten tube to lower its temperature to about 160 180F. Because ofthe type of second cooling means, further expansion of the bubble wascarried out to a desired blow ratio size of 3.5 ie during the secondexpansion step, the blow ratio was 1 .8 over and above the original blowratio.

The frost line of the material was approximately 26 inches above the dieand extrusion was carried out at about 160 feet per minute.

It was found that the product obtained had substantially no crystallinesites in the material, but rather, consisted of a crystallite populationsurrounded by amorphous areas. The crystallite population, due tostretching, was expanded to provide a more or less uniform product.

The characteristics of the product were tested and it was found that thehaze percentage, as measured by ASTM test D-1003-6l, was about 3.5%, thegloss percentage, as measured by ASTM test D523-53T was 128%, thetensile strength average (of the machine and transverse directions) was3250 psi, based on a machine direction strength of 3500 and a transversedirection strength of 3000, and an elongation percentage of 575%. Thesecant modulus of the film was 30,000 based on an average of machine andtransverse direc tion, and the film had an impact strength of 68 gramsper mil (based on a 1 mil sample). As will be seen such products possessvastly superior strength, optical and shrink characteristics compared toconventional prior art products, and the products were eminentlysuitable for high quality shrink film with substantially no distortionto printed indicia. Moreover, the product was also eminently suitablefor high gloss wrappings for special products e.g. bread, etc,

Additional tests were carried out using various types of ethylenecopolymers and it was found that increases in the shrink, gloss, haze,etc. could be obtained over conventional products.

It will be understood that various modifications can be made to theinvention without departing from the spirit and scope thereof.

Having thus described our invention, what we desire to claim and secureby letters patent is:

I. An apparatus for producing film by the blown tube process, whereinthe apparatus includes at least one extruder and at least one extrusiondie, the improvement comprising first and second cooling means, saidfirst cooling means adapted to cool a molten tube of material extrudedfrom said die, said first cooling means having a control chamber sizedto receive said molten extruded tube for cooling same after extrusionfrom said die, said first cooling means being adapted to be mounted inoperative downstream relationship to said die in the direction ofmovement of said film, said first cooling means adapted to cool saidmolten extruded tube to a temperature above the crystallizationformation temperature of the material but at a temperature at which saidtube remains in a molten condition and sufficient to permit theformation of crystallites in the material, an annealing chamber locatedsubstantially midway between said first and second cooling means andlocated downstream from said first cooling means in the direction ofmovement of said film, said annealing chamber being sized to receive thefilm from the first cooling means while still in a molten condition,means for providing a vacuum operatively associated with said annealingchamber to withdraw sensible heat from said film and to permit expansionof same to the extent necessary to form crystallites therein, saidsecond cooling means being in operative association with and locateddownstream from said annealing chamber in the direction of movement ofsaid film, said second cooling means also having a chamber sized toreceive said film from said annealing chamber for further cooling andsolidifying the annealed molten film, to thereby produce an amorphousfilm having crystallites dispersed throughout said film and which issubstantially free of crystal formation therein.

2.-An apparatus as defined in claim 1 wherein said first cooling meanscomprises means for directing a stream of cooling gaseous medium ontosaid molten film, said means being adapted to completely surround saidfilm and permit the passage of the film through said chamber.

3. An apparatus as defined in claim 2 wherein said second cooling meansis the same as said first cooling means, said second cooling means beingadapted to direct a source of high volume low to medium velocity gaseouscooling medium onto said film.

4. An apparatus as defined in claim 3 wherein said first cooling means,said annealing chamber and said second cooling means are mounted in anenclosed area surrounding a film extruded from said extrusion die.

5. An apparatus as defined in claim 1 wherein said annealing chamber isan enclosed chamber adapted to surround a film extruded from saidextrusion die.

6. An apparatus for producing film of thermoplastic material by theblown tube process, said apparatus comprising extrusion means, anextrusion die operatively associated with said extrusion means to permitthe extrusion of a film of material, means permitting the formation of abubble from said film of thermoplastic material, first and secondcooling means. said first cooling means located downstream from said diein the direction of movement of said bubble, saidfirst cooling meanshaving a chamber sized to receive said bubble in a molten conditionafter extrusion of the thermoplastic material from said extrusion die,said first cooling means adapted to permit a first expansion of saidbubble and to cool still in the molten condition, means for providing avacuum for a second expansion of the bubble operatively associated withsaid annealing chamber, said vacuum means adapted to withdraw sensibleheat from the bubble and to permit said second expansion of same to theextent necessary to form crystallites in the thermoplastic material,said second cooling means being spaced from said annealing chamber inthe direction of movement of said bubble and having a chamber sized toreceive the annealed bubble, and means operatively associated with saidchamber in said second cooling means to cause a third expansion of thebubble and to further cool and solidify said annealed molten bubble tothereby form an amorphous film having crystallites dispersed throughoutsaid film.

7. An apparatus as defined in claim 6 wherein said first cooling meanscomprises means for directing a stream of cooling gaseous medium ontosaid molten bubble, said means being adapted to surround said bubble andpermit the passage of the bubble therethrough.

8. An apparatus as defined in claim 7 wherein said second cooling meansis the same as said first cooling means, said second cooling means beingadapted to direct a source of high volume low velocity gaseous coolingmedium onto said tube.

9. The apparatus of claim 1 further characterized in that said firstcooling means are adapted to cool said molten tube to a temperaturebetween about 20F to about lF below the temperature of the materialduring extrusion and to create a first expansion of said tube.

It). The apparatus of claim 6 wherein said first cooling means comprisesmeans for cooling the molten tube to a temperature between about 20F. toabout l00F. below the temperature of the material during extrusion.

11. The apparatus of claim 6 additionally comprising means forcollapsing said tube upon solidification thereof.

12. An apparatus for producing film of extrudable thermoplastic materialby the blown tube process, said apparatus comprising extrusion means, anextrusion die operatively associated with said extrusion means to permitthe extrusion of a film of material. means permitting the formation ofabubble from said film of thermoplastic material, first and secondcooling means, said first cooling means located downstream from said diein the direction of movement of said bubble, said first cooling meanshaving a chamber sized to receive said bubble in a molten conditionafter extrusion of the thermoplastic material from said extrusion die,said first cooling means adapted to direct a stream of cooling gaseousmedium onto said molten extruded bubble and to permit a first expansionof said bubble, said first cooling means cooling said molten extrudedtube to a temperature between about 20F to about l00F below thetemperature of the material during extrusion whereby said tube remainsin a molten condition and forms crystallites in the material, anannealing chamber located substantially midway between said first andsecond cooling means, said annealing chamber being spaced from saidfirst cooling means in the direction of movement of said bubble andbeing sized to receive the bubble while still in a molten condition,means operatively associated with said annealing chamber for providing avacuum for a second expansion of the bubble, said vacuum meanswithdrawing a substantial portion of the stream of gaseous mediumdirected onto said molten bubble from said first cooling means, saidvacuum means adapted to withdraw sensible heat from the bubble and topermit said second expansion of same to the extent necessary to formcrystallites in the thermoplastic material, said second cooling meansbeing spaced from said annealing chamber in the direction of movement ofsaid bubble and having a chamber sized to receive the annealed bubble,and means operatively associated with said chamber in said secondcooling means to direct a further stream of cooling gaseous medium ontosaid annealed bubble, said means causing a third expansion of the bubbleand solidifying said annealed molten bubble to thereby form an amorphousfilm having crystallites dispersed throughout said film. 1: h a

1. An apparatus for producing film by the blown tube process, whereinthe apparatus includes at least one extruder and at least one extrusiondie, the improvement comprising first and second cooling means, saidfirst cooling means adapted to cool a molten tube of material extrudedfrom said die, said first cooling means having a control chamber sizedto receive said molten extruded tube for cooling same after extrusionfrom said die, said first cooling means being adapted to be mounted inoperative downstream relationship to said die in the direction ofmovement of said film, said first cooling means adapted to cool saidmolten extruded tube to a temperature above the crystallizationformation temperature of the material but at a temperature at which saidtube remains in a molten condition and sufficient to permit theformation of crystallites in the material, an annealing chamber locatedsubstantially midway between said first and second cooling means andlocated downstream from said first cooling means in the direction ofmovement of said film, said annealing chamber being sized to receive thefilm from the first cooling means while still in a molten condition,means for providing a vacuum operatively associated with said annealingchamber to withdraw sensible heat from said film and to permit expansionof same to the extent necessary to form crystallites therein, saidsecond cooling means being in operative association with and locateddownstream from said annealing chamber in the direction of movement ofsaid film, said second cooling means also having a chamber sized toreceive said film from said annealing chamber for further cooling andsolidifying the annealed molten film, to thereby produce an amorphousfilm having crystallites dispersed throughout said film and which issubstantially free of crystal formation therein.
 2. An apparatus asdefined in claim 1 wherein said first cooling means comprises means fordirecting a stream of cooling gaseous medium onto said molten film, saidmeans being adapted to completely surround said film and permit thepassage of the film through said chamber.
 3. An apparatus as defined inclaim 2 wherein said second cooling means is the same as said firstcooling means, said second cooling means being adapted to direct asource of high volume low to medium velocity gaseous cooling medium ontosaid film.
 4. An apparatus as defined in claim 3 wherein said firstcooling means, said annealing chamber and Said second cooling means aremounted in an enclosed area surrounding a film extruded from saidextrusion die.
 5. An apparatus as defined in claim 1 wherein saidannealing chamber is an enclosed chamber adapted to surround a filmextruded from said extrusion die.
 6. An apparatus for producing film ofthermoplastic material by the blown tube process, said apparatuscomprising extrusion means, an extrusion die operatively associated withsaid extrusion means to permit the extrusion of a film of material,means permitting the formation of a bubble from said film ofthermoplastic material, first and second cooling means, said firstcooling means located downstream from said die in the direction ofmovement of said bubble, said first cooling means having a chamber sizedto receive said bubble in a molten condition after extrusion of thethermoplastic material from said extrusion die, said first cooling meansadapted to permit a first expansion of said bubble and to cool still inthe molten condition, means for providing a vacuum for a secondexpansion of the bubble operatively associated with said annealingchamber, said vacuum means adapted to withdraw sensible heat from thebubble and to permit said second expansion of same to the extentnecessary to form crystallites in the thermoplastic material, saidsecond cooling means being spaced from said annealing chamber in thedirection of movement of said bubble and having a chamber sized toreceive the annealed bubble, and means operatively associated with saidchamber in said second cooling means to cause a third expansion of thebubble and to further cool and solidify said annealed molten bubble tothereby form an amorphous film having crystallites dispersed throughoutsaid film.
 7. An apparatus as defined in claim 6 wherein said firstcooling means comprises means for directing a stream of cooling gaseousmedium onto said molten bubble, said means being adapted to surroundsaid bubble and permit the passage of the bubble therethrough.
 8. Anapparatus as defined in claim 7 wherein said second cooling means is thesame as said first cooling means, said second cooling means beingadapted to direct a source of high volume low velocity gaseous coolingmedium onto said tube.
 9. The apparatus of claim 1 further characterizedin that said first cooling means are adapted to cool said molten tube toa temperature between about 20*F to about 100*F below the temperature ofthe material during extrusion and to create a first expansion of saidtube.
 10. The apparatus of claim 6 wherein said first cooling meanscomprises means for cooling the molten tube to a temperature betweenabout 20*F. to about 100*F. below the temperature of the material duringextrusion.
 11. The apparatus of claim 6 additionally comprising meansfor collapsing said tube upon solidification thereof.
 12. An apparatusfor producing film of extrudable thermoplastic material by the blowntube process, said apparatus comprising extrusion means, an extrusiondie operatively associated with said extrusion means to permit theextrusion of a film of material, means permitting the formation of abubble from said film of thermoplastic material, first and secondcooling means, said first cooling means located downstream from said diein the direction of movement of said bubble, said first cooling meanshaving a chamber sized to receive said bubble in a molten conditionafter extrusion of the thermoplastic material from said extrusion die,said first cooling means adapted to direct a stream of cooling gaseousmedium onto said molten extruded bubble and to permit a first expansionof said bubble, said first cooling means cooling said molten extrudedtube to a temperature between about 20*F to about 100*F below thetemperature of the material during extrusion whereby said tube remainsin a molten condition and forms crystallites in the material, anannealing chamber located substantiAlly midway between said first andsecond cooling means, said annealing chamber being spaced from saidfirst cooling means in the direction of movement of said bubble andbeing sized to receive the bubble while still in a molten condition,means operatively associated with said annealing chamber for providing avacuum for a second expansion of the bubble, said vacuum meanswithdrawing a substantial portion of the stream of gaseous mediumdirected onto said molten bubble from said first cooling means, saidvacuum means adapted to withdraw sensible heat from the bubble and topermit said second expansion of same to the extent necessary to formcrystallites in the thermoplastic material, said second cooling meansbeing spaced from said annealing chamber in the direction of movement ofsaid bubble and having a chamber sized to receive the annealed bubble,and means operatively associated with said chamber in said secondcooling means to direct a further stream of cooling gaseous medium ontosaid annealed bubble, said means causing a third expansion of the bubbleand solidifying said annealed molten bubble to thereby form an amorphousfilm having crystallites dispersed throughout said film.